Servovalves find a wide range of applications for controlling air or other fluid flow to effect driving or control of another part e.g. an actuator.
A servovalve assembly includes a motor controlled by a control current which controls flow to an air valve to control an actuator. Generally, a servovalve transforms an input control signal into movement of an actuator cylinder. The actuator controls e.g. an air valve. In other words, a servovalve acts as a controller, which commands the actuator, which changes the position of an air valve's (e.g. a so-called butterfly valve's) flow modulating feature.
Such mechanisms are used, for example, in various parts of aircraft where the management of air/fluid flow is required, such as in engine bleeding systems, anti-ice systems, air conditioning systems and cabin pressure systems. Servovalves are widely used to control the flow and pressure of pneumatic and hydraulic fluids to an actuator.
Conventionally, servovalve systems operate by obtaining pressurised fluid from a high pressure source which is transmitted through a load from which the fluid is output as a control fluid. Various types of servovalves are known—see e.g. GB 2104249, US 2015/0047729 and U.S. Pat. No. 9,309,900.
Electrohydraulic servovalves can have a first stage with a motor, e.g. an electrical or electromagnetic force motor controlling flow of a hydraulic fluid to drive a valve member e.g. a spool valve of a second stage, which, in turn, can control flow of hydraulic fluid to an actuator for driving a load. The motor can operate to position a moveable member, such as a flapper, in response to an input drive signal or control current, to drive the second stage valve member e.g. a spool valve.
Particularly in aircraft applications, but also in other applications, servovalves are often required to operate at various pressures and temperatures. For e.g. fast acting air valve actuators, relatively large flows are required depending on the size of the actuator and the valve slew rate. For such high flow rates, however, large valve orifice areas are required. For ‘flapper’ type servovalves, problems arise when dealing with large flows due to the fact that flow force acts in the direction of the flapper movement and the motor is forced to overcome the flow forces. For clevis-like metering valves such as described in U.S. Pat. Nos. 4,046,061 and 6,786,238, the flow forces, proportional to the flow, act simultaneously in opposite directions so that the clevis is balanced and centered. The clevis, however, needs to be big due to the requirement for bigger orifices to handle larger flows.
There is a need for a servovalve arrangement that can handle large fluid flows effectively, whilst retaining a compact design.
The present invention provides a servovalve comprising: a fluid transfer valve assembly comprising a supply port and a control port; a moveable valve spool arranged to regulate flow of fluid from the supply port to the control port in response to a control signal; and a drive member configured to axially move the valve spool relative to the fluid transfer assembly in response to the control signal to regulate the fluid flow; wherein the valve spool comprises a tubular member defining a conduit for fluid from the supply port and an opening in fluid flow alignment with the supply port such that fluid flows from the supply port into the tubular member conduit; the valve spool further comprising an aperture via which fluid flows out from the conduit; and wherein the valve spool is axially moveable between an open position in which a fluid flow path is formed between the aperture and the control port and a closed position in which fluid is blocked from flowing between the aperture and the control port.
Preferably, the fluid transfer valve assembly further comprises a return port in fluid communication with the control port when the spool is in the closed position.
The drive member is preferably an elongate member having a first end arranged for driving connection with a motor and a second end which is preferably a ball joint, arranged to engage the valve spool.
In a preferred embodiment, a plurality of apertures are provided in the circumference of the spool.
Preferred embodiments will now be described with reference to the drawings.